Structural investigation of iron phosphate glasses using molecular dynamics simulation

The current study reports the first molecular dynamics models of iron phosphate glasses. Models were made for xFeO-(100 − x)P₂O₅ glasses with x = 30, 40 and 50 and for xFe₂O₃-(100 − x)P₂O₅ glasses with x = 30 and 40. This study also looks at the effects of mixed Fe²⁺/Fe³⁺ contents. The models are in good agreement with experimental results for nearest-neighbour distances and coordination numbers, and in reasonable agreement with X-ray and neutron diffraction structure factors. As expected the models contain a tetrahedral phosphate network with P-O distances of 1.50 ± 0.01 Å. The network connectivity is dominated by the expected Qⁿ (where n is the number of bridging oxygen) corresponding to the O:P ratio. These are average Qⁿ of 2.3 for 40FeO and 1.0 for 40Fe₂O₃ glasses respectively. Interestingly a small amount of non-network oxygen is found to be present in the 40Fe₂O₃ glass model. The Fe-O coordination is close to 4.5 in both FeO and Fe₂O₃ glass models, with Fe-O bond lengths of 2.12 Å and 1.89 Å respectively. The greater durability of xFe₂O₃-(100 − x)P₂O₅ glasses can be attributed to the lower content of P-O-P bonds and higher bond valence across Fe-O-P bonds. For 40Fe₂O₃ glass the Fe-Fe correlation shows a main peak at 5-6 Å in good agreement with the result from magnetic scattering which was interpreted in terms of speromagnetic order.